Re: [PATCH v2 4/4] util/event-loop-base: Introduce options to set the thread pool size

[Nicolas Saenz Julienne <nsaenzju@redhat.com>]

On Mon, 2022-03-14 at 13:35 +0000, Stefan Hajnoczi wrote:
> On Fri, Mar 11, 2022 at 11:40:30AM +0100, Nicolas Saenz Julienne wrote:
> > On Thu, 2022-03-10 at 10:45 +0000, Stefan Hajnoczi wrote:
> > > On Thu, Mar 03, 2022 at 04:13:07PM +0100, Nicolas Saenz Julienne wrote:
> > > > @@ -537,10 +546,19 @@
> > > >  #                 0 means that the engine will use its default
> > > >  #                 (default:0, since 6.1)
> > > >  #
> > > > +# @thread-pool-min: minimum number of threads readily available in the thread
> > > > +#                   pool
> > > > +#                   (default:0, since 6.2)
> > > > +#
> > > > +# @thread-pool-max: maximum number of threads the thread pool can contain
> > > > +#                   (default:64, since 6.2)
> > > 
> > > Here and elsewhere:
> > > s/6.2/7.1/
> > 
> > Yes, forgot to mention it was a placeholder, as I wasn't sure what version to
> > target.
> > 
> > > > @@ -294,6 +314,36 @@ void thread_pool_submit(ThreadPool *pool, ThreadPoolFunc *func, void *arg)
> > > >      thread_pool_submit_aio(pool, func, arg, NULL, NULL);
> > > >  }
> > > >  
> > > > +void thread_pool_update_params(ThreadPool *pool, AioContext *ctx)
> > > > +{
> > > > +    qemu_mutex_lock(&pool->lock);
> > > > +
> > > > +    pool->min_threads = ctx->thread_pool_min;
> > > > +    pool->max_threads = ctx->thread_pool_max;
> > > > +
> > > > +    /*
> > > > +     * We either have to:
> > > > +     *  - Increase the number available of threads until over the min_threads
> > > > +     *    threshold.
> > > > +     *  - Decrease the number of available threads until under the max_threads
> > > > +     *    threshold.
> > > > +     *  - Do nothing. the current number of threads fall in between the min and
> > > > +     *    max thresholds. We'll let the pool manage itself.
> > > > +     */
> > > > +    for (int i = pool->cur_threads; i < pool->min_threads; i++) {
> > > > +        spawn_thread(pool);
> > > > +    }
> > > > +
> > > > +    while (pool->cur_threads > pool->max_threads) {
> > > > +        qemu_sem_post(&pool->sem);
> > > > +        qemu_mutex_unlock(&pool->lock);
> > > > +        qemu_cond_wait(&pool->worker_stopped, &pool->lock);
> > > > +        qemu_mutex_lock(&pool->lock);
> > > 
> > > Same question as Patch 1. This looks incorrect because qemu_cond_wait()
> > > already drops pool->lock if it needs to block.
> > 
> > Yes, I'll fix that.
> > 
> > > Also, why wait? If worker threads are blocked for some reason then our
> > > thread will block too.
> > 
> > Exiting thread_pool_update_params() before honoring the new constraints is a
> > source of potential race conditions (having to worry for situations where
> > cur_threads > max_threads), and on systems where determinism is important it's
> > crucial to have a clear boundary between 'unsafe' and 'safe' states.
> 
> On the other hand it creates a reliability problem where a random worker
> thread can block all of QEMU. Maybe it's better to let a blocked worker
> thread terminate eventually than to hang QEMU?

OK, fair enough. I'll switch to that behaviour.

Thanks!

-- 
Nicolás Sáenz